Delamination Problems of Parylene Conformal Coatings
Providing a uniform and pinhole-free substrate coating that is ultra-thin, lightweight and durable, parylene coatings completely conform to targeted components and assemblies. Parylene CVD generates a structurally continuous film that, with appropriate pre-treatment, penetrates deep within substrate surfaces, rather than simply attaching themselves to substrates as liquid-application coatings do. These provide effective, dielectrically efficient safeguards with coatings as thin as a fraction of a micrometer. Parylene is chemically and biologically inert and stable, an excellent barrier material to abrasive chemicals, bodily fluids, solvents, liquid water and water vapor.
Printed circuit boards (PCBs) and similar assemblies with specialized component configurations – angular surfaces, crevices, exposed internal surfaces, flat facades, pointed or sharp edges --benefit from parylene’s all-inclusive conformal protection. The process is eminently repeatable and controllable, delivering extremely consistent results from batch-to-batch.
However, despite its superiority as a conformal coating, parylene is not flawless. Its overlying chemical structure may restrict dependable interface, limiting adhesion with some substrates. The CVD process that sources the majority of parylene's advantages as a conformal coating simultaneously nullifies chemically-based substrate adhesion; only mechanical adhesion is possible.
In these cases, delamination can emerge as a problem for parylene coated surfaces. Delamination occurs in cases where the conformal coating separates from the covered surface, producing a poor, unacceptable finish by lifting away from the substrate, resulting in a torn, unattached, and non-conformal coating. Although surface exposure may not be complete, delamination uncovers at least some segment of the region to-be-protected, entirely defeating the purpose of conformal coating.
Delamination is one of the worst outcomes confronting the use of parylene. Partial lifting of the parylene coating is sufficient to qualify as delamination. Either standard or corrective processes -- such as demasking or a reaction to production materials -- can instigate delamination. Care must be taken both prior to and during CVD application procedures to assure subsequent delamination episodes do not occur. Post-production and inspection procedures must similarly target the possibility of delamination, an extremely negative outcome that must be avoided or identified and corrected.
Sources of Parylene Delamination
Factors that influence delamination include:
- Materials incompatibility: The parylene coating and the substrate surface to-be-adhered-to need to bond together. Incompatibility between the parylene and the surface to-be-covered generates an incongruity of surface energies where the parylene and substrate meet; in these cases, only minimal bonding occurs, if it develops at all, frequently leading to delamination.
- Coating porosity: A difference in vapor pressure develops in the region between the parylene coating and the surface, causing a susceptibility to moisture intrusion and permeation through to the PCB. Consequent fluctuations of temperature and pressure generate osmotic pressures that can separate the coating from the substrate.
- Surface cleanliness: Above all, a clean surface is necessary for adhesion. Contaminated surfaces do not support adhesion and are conducive to delamination.
Taking these conditions affecting the adhesion between parylene and the substrate as the foundation of subsequent delamination issues generates solutions to the problem.
Preventing Parylene Delamination
Delamination can be prevented by enacting the following techniques prior to-, during, and post-CVD processing:
- Assuring materials compatibility: Appropriate coordination between the grade of parylene conformal coating and the substrate material generates reliable adhesion and lamination. It may be necessary to change either the coating type or modify the surface energy. The objective is transform the interaction of surface energies so they better support adhesion.
- Moisture permeability: Selecting a parylene type exhibiting appropriate moisture impermeability while maintaining materials compatibility with the substrate is necessary.
- Surface cleanliness: Contamination – dirt, mold release agents, process residue, etc. – should be removed from components before application. Cleaning the PCB enhances parylene’s adhesion/laminate and surface energy qualities.
Materials selection must be connected to an assembly’s composition and uses. Parylene C’s elongation-to-break factor is superior to either types D or N, suggesting enhanced delamination properties, but each coating job will have specialized factors to consider.
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